RF Envelope Tracking Yields PA Efficiency Breakthrough

Feb. 14, 2012
Envelope tracking improves RF power amplifier efficiency solving multiple problems in basestations and cellphones.

Fig 1. In today’s handsets, a dc-dc converter with a fixed output voltage powers the PA. While the PA is linear, the efficiency is low. With Nujira’s NCT-L1100 envelope-tracking tracking chip, the baseband signal drives the chip and varies the dc power supply so the PA is maintained at or near the compression point, resulting in far greater efficiency.

Fig 2. Nujira’s NCT-T9102 RF PA characterization system provides a way to test PAs with and without envelope tracking. The company developed it as a result of the dearth of testing and characterization test options from existing instrument vendors.

The Coolteq.L NCT-L1100 envelope-tracking (ET) power-supply modulator from Nujira Ltd. promises to improve power-amplifier (PA) efficiency, reducing power consumption and heat in basestations and smart phones.

PA efficiency is a critical factor in the RF design of all modern wireless systems. In cellular basestations, power consumption costs carriers millions of dollars annually. In smart phones, PA efficiency is an increasing concern as battery life declines and handsets get hotter.

This inefficiency is brought about because the most recent higher-speed 3G and 4G technologies use modulation methods such as WCDMA and Long-Term Evolution (LTE) with quadrature amplitude modulation (QAM) over orthogonal frequency-division multiplexing (OFDM). All of these technologies require linear PAs that are less efficient by their very nature.

The typical linear RF PA operates in class AB to achieve its linearity. Maximum theoretical efficiency is 50%, but in practice, maximum efficiencies are in the 30% to 35% range. This efficiency is best achieved when the amplifier is in compression or operating near the compression point.

Compression occurs when the input signal is at or near its peak. With the latest modulation methods, these peaks occur in short bursts rather than continuously. The PA then operates well below the compression point for much of the transmission, providing good linearity with an efficiency average of 20% or less. This causes the power dissipated as heat to increase, and the excessive current drawn by the PA leads to shorter battery life.

Envelope tracking deals with this problem by replacing the usual fixed dc supply with a fast-changing dc supply that dynamically tracks the amplitude or envelope of the RF signal. The NCT-L1100 implements this function. The signals from the smart phone’s baseband chipset are sent to the ET power supply chip (Fig. 1).

This varies the dc supply voltage to the PA over a 1- to 5-V range, keeping the PA in or very near saturation. During this time, the overall power efficiency jumps for a previous best of about 30% to about 60%. At or near compression, more of the power is sent to the antenna, and less is dissipated as heat.

The NCT-L1100 works over a wide frequency range and supports LTE’s 20-MHz channel bandwidth. It also delivers superior adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM) performance. The chip uses the new industry-standard OpenET analog envelope interface as well as the MIPI RF front-end control interface.

The ET approach also solves another problem. Most smart phones are multi-mode and multi-band, so they must handle the higher speeds of LTE but also be compatible with older 3G modes such as WCDMA and HSPA. Furthermore, they must work over multiple bands covering from about 700 MHz to over 2700 MHz.

Typical phones cover about six or seven of those bands, meaning there usually is a single PA for each band. While a single broadband PA can be used, it is very inefficient. ET technology makes multi-mode and multi-band PAs viable.

For example, a single ET-type PA can cover from 700 to 950 MHz with an efficiency in the 50% to 60% range. Another ET PA then can be used to cover the 1400- to 2100-MHz range, and a third can cover 2300 to 2700 MHz. This greatly reduces the total number of PAs in addition to the cost, printed-circuit board (PCB) space, and overall power consumption.

Envelope tracking also helps deal with high voltage standing wave ratios (VSWR) resulting from the antenna detuning common in handsets. Poorly matched and greatly detuned antennas can produce SWR values as much as 20:1. Most PAs can’t deal with this range, even with variable tuning networks. High VSWR leads to a greatly increased error vector magnitude (EVM), which results in lower speeds or dropped calls. ET PAs deliver consistent saturated operation, keeping the EVM low even with higher VSWR.

Also, envelope tracking will allow the use of CMOS PAs in handsets. CMOS PAs are less expensive than the typical gallium-arsenide (GaAs) PAs used today. They work well in GSM and Wi-Fi applications, but their compression characteristics make them less desirable in HSPA and LTE applications.

An ET chip can eliminate this weakness, though, making CMOS PAs a good design option. This benefit potentially can lower overall PA costs and provide a gateway opportunity for single-die CMOS integration of the PA and the ET circuitry.

Nujira’s chips will be available later this year, but most current PAs aren’t yet set up to accommodate them. Most PAs have a built-in dc supply bypass capacitor that prevents the ET circuitry from working. In addition, many LTE baseband chips lack the necessary signal output connections to send the signal to the PA. Nujira is working with baseband and PA vendors to solve this problem.

Finally, Nujira has introduced an RF PA characterization system (Fig. 2). The company developed it to design and test its own chips since nothing else was up to the task. Designated the NCT-T9102, the system provides a complete way to test all aspects of a PA with envelope tracking.

The NCT-T9102 features a high-bandwidth, low-impedance dynamic supply modulator with a power supply bandwidth in excess of 60 MHz, incorporating accurate measurement of dynamic PA efficiency. The supplied software provides real-time displays of the AM/AM and AM/PM distortions in the amplifier as well as the resultant spectrum and efficiency profile.

The hardware includes an IQ modulated transmitter and two complex receivers, covering the 650- to 2700-MHz frequency range. The system also includes a selection of predefined WCDMA and LTE waveforms, as defined by the OpenET Alliance, and enables development of the shaping table function that maps RF level to PA supply voltage.

Additionally, Nujira makes ET hardware for both wireless basestations and TV broadcast stations. The company will show both the NCT-T9102 test system and the Coolteq.L NCt-L1100 ET chip at Mobile World Congress in Barcelona, February 27 to March 1.

Nujira Ltd.

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